This invention relates to bicycles with frames adapted for the routing of control cables through the interiors of frame members. More particularly, it relates to a frame head tube portion, frame configuration and cable guidance elements which provide well sealed and direct internal control cable paths characterized by short cable lengths, low cable bending and the absence of outer cable (cable jacket or housing) passing through the frame. The invention obtains the benefits of control device actuation having low elasticity and low friction.
An example of the prior art in the internal cabling of bicycles is shown in U.S. Pat. No. 4,484,756 (Takamiya et. al.), where bosses are provided on the side of the main frame of a bicycle so that a jacketed cable (co-axial inner and outer cables) can be run through the frame from an entrance point behind the head tube to an exit point forward of the seat tube. U.S. Pat. No. 4,513,986 (Trimble) shows three internal cables inserted in a monocoque frame at locations behind the head tube. U.S. Pat. No. 4,585,246 (Diekman and Dietz) shows a jacketed cable which enters a top tube lug at a point behind the head tube and exits a seat post bracket at a point forward of the seat tube. U.S. Pat. No. 4,565,383 (Isaac) shows a rear derailleur inner cable that enters a chainstay behind the bottom bracket and exits through a rear dropout at a point forward of the rear axle. U.S. Pat. No. 4,647,060 (Tomkinson) shows control cabling entering the elongated frame member of a low profile bicycle at locations rearward of the headset bearings.
Areas in which control cabling can be improved are indicated by examination of factors which affect cable performance. An ideal cable would prefectly transmit motion from the lever which pulls it to the device it actuates. Elasticity detracts from this ideal by causing a loss in relative motion along the length of the cable. Friction detracts by causing a loss of transmitted force. Respectively, the effects of elasticity and friction are reflected in the terms `spongy` and `stiff`, which are sometimes used to describe the imperfect feel of a control actuation.
Given a particular inner cable, the amount of elastic stretching caused by an applied tensile force is directly proportional to the cable length, which should thus be minimized. Because outer cable is also elastic and considerably more so than the frame tubes of a bicycle, a cable path will preferentially utilize frame parts rather than outer cable to provide that compressive resistance which must necessarily counter the tension of an inner cable.
Cable friction and attendant wear occur principally at cable bends, the magnitude of the friction increasing with the bend angle and decreasing with the bend radius. To reduce friction, it is thus desirable to eliminate bend locations, to reduce the bend angle where a cable path must turn, and to provide large curvature radii for necessary bends. Lubrication and the presence of fouling contaminants are also important factors, as are burrs, edges or crimping at transitions. A cable can bind tight in a section of housing if sufficient dirt and rust are present. Corrosive contamination is particularly severe in the vicinity of conventional bottom brackets and chain-stays. Further considerations with respect to internal cabling include the aesthetic appearance and aerodynamic effects of exposed cable sections and cable fittings.